1. Field of the Invention
The present invention relates to a customer premises optical network termination unit installed in a subscriber's premise in a PON (Passive Optical Network) type optical transmission system.
2. Related Art
The PON-type optical transmission system in which optical transmission paths connected to a center are laid to a plurality of subscriber's premises by splitting the paths by passive-type splitters is employed in optical networks such as FTTH and CATV as described in Japanese Patent Application Laid-open No. Hei.9-214541 as described below. This type of optical transmission system is also called as PDS (Passive Double Star).
As shown in FIG. 8, in the PON-type optical transmission system, an optical line terminal (OLT) 901 of the center is connected to a multiplexing terminal of the optical splitter 903 through an optical fiber 902 and optical network units (ONU) 905-1, . . . and 905-n installed in the respective subscriber's premises are connected to a plurality of demultiplexing terminals of the optical splitter 903 through optical fibers 904-1, . . . and 904-n. 
FIG. 9 shows a structure of the ONU 905-1 as one example among the ONUs 905-1, . . . and 905-n. The ONU 905-1 is composed of an optical transceiver section 906, an ONU functioning section 907, a serial/parallel converting section (SERDES) 908 and Ethernet (registered trademark) interface 909. The optical fiber 904-1 connected to the optical splitter 903 is connected to the optical transceiver section 906 via an optical termination unit 910 provided in each subscriber's premise.
10/100/1000 BASE-TXs and the like may be used as the Ethernet interface 909 and a LAN cable 911 for example is connected thereto. Terminal equipments such as computers and printers may be directly connected to the LAN cable 911 or switches such as a router 912 and a switching hub 913 may be connected to the LAN cable 911. Then, different terminal equipments may be connected to those switches. Still more, using a MSA (Multi-Source Agreement) interface module accommodated to specifications of the MSA interface as the interface 909 brings about such considerable merits that those equipments may be directly connected to MSA interface slots provided in an external node and it becomes unnecessary to connect such LAN cable, thus cutting a space for that.
A structure of the ONU provided with the MSA interface module will be explained in detail below with reference to FIG. 10. FIG. 10 is a block diagram showing one exemplary detailed structure of the ONU 905 (either one of the ONUs 905-1 through 905-n) and has the MSA interface module as the interface 909. The optical transceiver section 906 converts an optical signal into an electrical signal or an electrical signal into an optical signal transmitted between the OLT 901 and the customer premises external node and processes a physical layer (L1) in the OSI (Open System Interconnection) reference model. In order to control and administrate the optical/electrical conversion and electrical/optical conversion processes, the ONU 905 is provided with a management processing section 921.
The management processing section 921 is connected with the optical transceiver section 906, a temperature sensor not shown and others and receives physical data such as optical output of a laser diode (LD) built in the optical transceiver section 906 and temperature of the ONU 905. The MSA interface module 909 is also provided with serial ports 909b accommodating an I2C (Inter Integrated Circuit) and the management processing section 921 is connected with the serial ports 909b by I2C signal lines. Thereby, the external node connected with the MSA interface module 909 can receive the abovementioned physical data through the serial ports 909b. 